Patent Reference 1 describes a method for condensing a water-containing organic substance, which comprises a combination of rough dewatering in a distillation column and subsequent VP dewatering, as shown in FIG. 2. According to the method, the overhead vapor that has been formed through concentration of a water-containing organic substance to the azeotropic composition thereof in a distillation column is all condensed to liquid, and a part of the condensed liquid is fed to a VP unit. In the VP unit, first, the supplied liquid is wholly vaporized in a vaporizer at a high temperature at which the VP driving force is large, and the vapor is dewatered through a membrane. Heat must be given to the vaporizer by a heated vapor externally applied thereto, and the dewatered product vapor is fed to the reboiler at the bottom of the distillation column and condensed to recover the heat.
The method is characterized in that, even when the distillation column is designed for normal pressure or reduced pressure under which a water-containing organic substance can be readily concentrated to a high concentration (the azeotropic concentration thereof is high), and without being bound by it, the pressure of the vapor to be introduced into the VP unit can be increased to a high pressure by elevating the vaporization temperature and the VP driving force can be thereby increased to reduce the membrane area.
As described above, the latent heat of the product vapor must be recovered for the heat source of the reboiler at the bottom of the distillation column in order not to detract from the energy-saving merit, but the distillation column part and the VP part could not always be well balanced in point of the quantity of heat and, in addition, when the operation load fluctuates, the two would mutually affect each other, therefore causing a problem in the operation flexibility. Accordingly, a system is desired that secures operation of the distillation column part and the VP part with highest efficiency.
In relation to the problem with the method of Patent Reference 1, a technique for energy saving and efficiency enhancement of a rectifier part is described in Patent Reference 2. This is a distillatory apparatus for water-containing alcohol, comprising multiple rectifiers (10) and (11) that are operated under different pressures, as shown in FIG. 4, and is a system in which the overhead vapor of the high-pressure rectifier (10) is used as the heat source for the low-pressure rectifier (11). In case where the rectifier part is designed for energy saving as in this, the heat recovered from the VP part does not serve any purpose, and therefore it is highly necessary to enhance efficiency inside the region of the VP part.
FIG. 5 in Non-Patent Reference 1 (see FIG. 5 attached here) shows one example of a flow diagram of a combination of a distillation column and VP. In this flow, the vapor not condensed in the reflux condenser of the distillation column overhead vapor is fed to the VP part while kept as vapor. In case where the operation pressure in the distillation column is designed high, the system could be simple with high efficiency; but in case where a VP unit is arranged later in the distillation column designed for atmospheric pressure or reduced pressure (in vacuum), the pressure on the primary side of the VP unit must be atmospheric pressure or in vacuum, and as a result, the VP driving force may be small, a large membrane area may be necessary and sufficient dewatering could not be attained. Accordingly, a system for the VP part not bound by the designed pressure of the distillation column is important.
In case where water is removed to a high degree from a dilute aqueous solution of an organic substance such as ethanol produced through fermentation, and when the aqueous solution is directly fed and processed through membrane separation (especially VP), then the membrane area may be too large. Therefore, it is widely known by those skilled in the art that the solution is generally concentrated first in a distillation column to a range in which it can be concentrated reasonably (in a system where an azeotrope with water is formed, the solution is concentrated to a concentration somewhat lower than the azeotropic composition), followed by VP-dewatering, and the process of this type is favorable from the viewpoint of the equipment cost and the operating cost. In such a case, for enhancing the efficiency in the VP part without being influenced by the designing condition for the distillation column, all the overhead distillate in the distillation column is condensed into a liquid and the resulting liquid is fed to the finishing VP part, as shown in FIG. 2 (Patent Reference 1), and the method is one extremely effective method. In investigating the enhancement of the efficiency in the VP part, when the starting material supply mode to the VP part is limited to the case where the material is fed thereto in the form of a liquid, then the efficiency in the distillation part can be enhanced according to the method shown in Patent Reference 2 or the like and the efficiency of the VP part can be enhanced by applying thereto the present invention to be mentioned hereinunder, and consequently the efficiency of both the two can be enhanced basically with no mutual interference with each other. There are many application cases where the starting material is liquid, such as a case where dewatering is attained in an existing water-containing product distillation column arranged later or a case of dewatering and repurification of solvent used in precision washing in an electronic industry, etc., and therefore the industrial applicability of the invention of enhancing the efficiency of the VP system for a liquid starting material is great.
An example of the constitution of the VP part for a case where the starting material is liquid is shown as the VP part in FIG. 2 (Patent Reference 2), and for clarifying the difference between the constitution thereof and the constitution of the present invention, a basic constitution of an already-existing liquid material VP is shown separately in FIG. 3.